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Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes: Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK


The Cardigan Bay Basin (UK) may have functioned as a deep and narrow strait, and thereby influenced Early Jurassic oceanic circulation through the northern and southern Laurasian Seaway, and between Boreal and Peri-Tethys domains. Toarcian hemipelagic deposits of the basin in the Mochras borehole show strongly bioturbated contourite facies. Trace fossils are strongly dominated by Phycosiphon incertum (represented by four morphotypes), which was produced by opportunistic colonizers. Thalassinoides, Schaubcylindrichnus and Trichichnus are common (the latter is a deep-tier trace fossil produced by filamentous sulfide-oxidizing bacteria with a high tolerance for dysoxia), accompanied by less common Zoophycos, Planolites, Palaeophycus, Teichichnus, Rhizocorallium, Chondrites, and dwelling and resting structures, such as cf. Polykladichnus, Siphonichnus, Skolithos, Arenicolites, Monocraterion and Lockeia. Ichnological and lithological signals suggest repetitive fluctuations in benthic conditions attributed to a hierarchy of orbital cycles (precession and obliquity [4th order], short eccentricity [3rd order], long eccentricity [2nd order] and Earth–Mars secular resonance [1st order]). The Pliensbachian–Toarcian transition appears to be a significant palaeoceanographic turning point in the Cardigan Bay Basin, starting a CaCO3 decline, and with the most severe oxygen crisis of the Tenuicostatum Zone (here dysoxic but not anoxic) ending at the onset, in the early Serpentinum Zone (Exaratum Subzone), of the Toarcian negative carbon isotope excursion (To-CIE—linked with the Toarcian Oceanic Anoxic Event occurring in the lower part in the Serpentinum Zone). This trend contrasts with the prevalence of anoxia synchronous with the To-CIE in many other settings. Minor dysoxia returned to the Mochras setting in the latest Thouarsense to Dispansum zone interval. Extreme climate warming during the To-CIE may have enhanced and caused a reversal in the direction of deep marine circulation, improving oxygenation of the sea floor. Spectral analysis of binary data on ichnotaxa appearances gives high confidence in orbital signals and allows refined estimation of ammonite zones and the duration of the Toarcian (minimum ~ 9.4 Myr).

1 Introduction

The impact of orbital forcing versus internal Earth processes on climate change through Earth history is an important topic of debate (e.g., Lourens 2021). Here, the influence of these factors on the Earth system in the deep geological past (the Toarcian age, Early Jurassic epoch), over the course of about 10 Myr, is discussed in relation to major climate changes and deep-sea circulation. The Llanbedr (Mochras Farm) borehole, hereafter referred to as Mochras, is situated in the Cardigan Bay Basin (Wales, UK) and cored an unusually thick (~ 1300 m) Early Jurassic age succession, with relative completeness of ammonite chronozones (Woodland 1971). During the Early Jurassic, this extensional basin was located at a mid-palaeolatitude (c. 35–40°N), on the NW fringe of the European shelf and within the Laurasian Seaway (Fig. 1). The Toarcian part of the core has been studied in many respects, including biostratigraphy (Woodland 1971; Page in Copestake and Johnson 2014; McArthur et al. 2016), lithology (Woodland 1971; Dobson and Whittington 1987; Hesselbo et al. 2013; Copestake and Johnson 2014; McArthur et al. 2016), carbon cycle and climate-driven local sedimentary processes (Jenkyns et al. 2002; Xu et al. 2018a), ostracod and foraminiferal assemblages (Boomer 1991; Copestake and Johnson 2014; Reolid et al. 2019; Rodríguez-Tovar et al. 2020), magnetostratigraphy (Xu et al. 2018b), and calcareous nannofossils and primary productivity (Bown 1987; Menini et al. 2021; Paulsen and Thibault 2023). A chemostratigraphic reference record shows a major 7‰ negative excursion in δ13CTOC, associated with the Toarcian Oceanic Anoxic Event (T-OAE) preceded and followed by early Toarcian positive excursions (Jenkyns et al. 2002; Xu et al. 2018b). Climate-driven, accelerated continental weathering and hydrological cycling was suggested as the cause of intensified sediment supply to the Cardigan Bay Basin (Xu et al. 2018a, b). However, a more focussed characterization of sedimentary environment, ichnology, and benthic life conditions has not yet been published.

Fig. 1
figure 1

Location maps. a Toarcian palaeogeography with localization of the Laurasian Seaway and Mochras borehole. b Enlargement of the area in the frame a showing major basins and elevated areas in the Toarcian times (after Cope et al. 1992, amended) and enhanced thermohaline circulation from the North from the late Falciferum Subzone on, with generally improved oxygenation of the bottom (circulation intensity and average oxygenation achieved its highest level in the late Bifrons and Variabilis to earliest Thouarsense zones. CBB Cardigan Bay Basin. c The same map showing the inferred current flow paths during the Tenuicostatum Zone–Exaratum Subzone and), the latest Thouarsense–Dispansum Zone and (to a lesser extent) Pseudoradiosa Zone thermal anomalies related to a strengthening of warm equatorial Tethyan westward currents and associated with the conspicuous ichnodiversity/oxygenation crisis in Mochras. Note the pivotal position of the Cardigan Bay Strait (now Cardigan Bay Basin – CBB) in the Laurasian Seaway between Boreal Sea and Peri-Tethys domain

The prominent expression of Carbon Isotope Excursions (CIEs) in Mochras with an amplitude of about one per mil in δ13CTOC (Xu et al. 2018a) testifies to the expanded and relatively complete stratigraphic record therein, particularly in the early Hettangian and Sinemurian age part of the section, allowing for a distinction between intrinsic Earth processes and extrinsic solar system dynamics as the driving mechanisms for the Early Jurassic δ13C fluctuations (Xu et al. 2018a; Storm et al. 2020; Hollaar et al. 2023; Hesselbo et al. 2023).

The duration of the whole Toarcian, based on astronomical calibration is not well constrained to date. Time series of magnetic susceptibility (Huret et al. 2008) and XRF-derived Ti content from the Sancerre borehole, Paris Basin, France, has been used, which yielded a minimum duration of 8.3 Myr from 405-kyr long eccentricity cycles (Boulila et al. 2014, 2019), a result largely confirmed by Huang (2018). Nonetheless, numerous studies of the T-OAE CIE interval itself have not yet resulted in a consensus on duration, with estimates varying widely. It has been estimated to be ~ 300 to ~ 500 kyr (Kemp et al. 2005, 2011; Boulila et al. 2014, 2019; Boulila and Hinnov 2017; Ikeda et al. 2018) or ~ 900 kyr (Suan et al. 2008; Huang and Hesselbo 2014).

In this paper, detailed sedimentological and ichnological data are presented. They allow recognition of ecological parameters, as trace fossils are sensitive in situ indicators of even very subtle environmental changes such as current strengths, oxygen content, food supply, and stability of the environment, allowing for new interpretations and correction of existing ones, based on other proxies. Furthermore, deep-sea environmental changes using both new and existing data (δ13CTOC, calcium content, organic geochemistry, benthic fossils—Xu et al. 2018a; Reolid et al. 2019) are discussed to determine which changes are caused by intrinsic Earth processes and which can be attributed to extrinsic solar system dynamics, also based on spectral analyses of trace fossil appearances. Benthic invertebrates respond to various environmental changes driven by orbital forcing (e.g., Wetzel 1991; Erba and Premoli Silva 1994; Hüneke and Stow 2008; Pervesler et al. 2008; Rebesco et al. 2014), although the mechanisms of control are still poorly understood. In the present study we document and analyse lithological and ichnological cycles that can be related to orbital controls, and by this means provide a new estimate of the duration of the Toarcian and ammonite zones within this stage.

2 Material and methods

The Mochras slabbed core (in 1 m core sections) is held at the British Geological Survey, Keyworth, Nottingham, UK. A nearly continuous 261 m-thick section of core was investigated using standard visual sedimentological description at centimetre scale accuracy (Fig. 2a, b) with special attention paid to the integration of sedimentological and ichnological observations. Depths in Figs. 2a, b and 3 are referred to metres below surface (mbs). Alternative descriptive classifications were adopted—for siliciclastic sections, the classical array of claystone-mudstone-siltstone-sandstone-conglomerate was used, while in sections with high carbonate content (over 30–40%) the classification of carbonate rock lithotypes based on Dunham (1962) was used (mudstone-wackestone-packstone-grainstone; Figs. 2a, b and 4, 5, 6). Based on ammonite occurrences intervals have been assigned to chronozones as documented most recently in McArthur et al. (2016); here we follow convention and use roman type to denote chonozones (see discussion of distinction between biozones and chronozones in Jurassic stratigraphy in Hesselbo et al. 2020a).

Fig. 2
figure 2figure 2

Investigated part of the Mochras Borehole showing stratigraphy (stages, ammonite zones, and subzones), lithology, ichnology and geochemistry of the Toarcian section. a Tenuicostatum and Serpentinum zones. b Bifrons–Aalensis zones. O, other, rarely occurring trace fossils: Ar, Arenicolites; Ast, Asterosoma; Ch, Chondrites; Lo, Lockeia; Mon, Monocraterion; Pol, cf. Polykladichnus; Rh, Rhizocorallium; Sip, Siphonichnus; Sk, Skolithos. Trichichnus (filamentous mat-forming, sulfide-oxidizing bacteria and “pyritized tubes”) represent traces of unidentified life activity/disputable origin. Ub—micrite/mudstone of ‘massive’ appearance, with undulated or cuboid parting. Geochemistry: δ13CTOC—carbon isotope curve (after Xu et al. 2018a); XRF-derived calcium record (after Xu et al. 2018a). Cyclicity: thin dashed lines—4th order (c. 20 kyr) cycles; solid gray lines—3rd order (c. 100 kyr) cycles; red lines—2nd order (c. 405 kyr) cycles; grey bar on the right—1st order (c. 2.5 Myr) cycles expressed in bottom oxygenation (shaded) and ichnology. Foraminiferal biotic crisis after Reolid et al. (2019). Red bars in the metric depth column—position of the sections presented in Fig. 4. Biostratigraphy from Page in Copestake and Johnson (2014) and McArthur et al. (2016) as summarised in Additional file 3

For numerical cyclicity (spectral) analysis, occurrences of trace fossils Phycosiphon morphotypes 2, 3, Thalassinoides, Schaubcylindrichnus, Planolites and Trichichnus (Figs. 2a, b, 5, 7 and 8), as well as intervals with preserved lamination and undulated bedding (see below) were used to compile discrete binary time series. Ranges of trace fossils, undulated bedding and lamination were digitized by visual sampling with an even step of 10 cm. Resultant time series consist of approximately 2550 binary digits each (Additional file 1).

To allow comparison between the ichnological record and existing δ13CTOC, TOC content, Ca content, Zr/Rb, Ti/Al, Si/Al and gamma ray records, curves based on smoothed data are juxtaposed (Fig. 3). Ichnological records were converted to curves by applying a smoothing spline algorithm (De Boor 2001) directly to binary time series with smoothing factor values in the 8–9 range, in each case experimentally adjusted to obtain an optimal degree of smoothing.

Fig. 3
figure 3

© UKRI 2024

Smoothing spline-smoothed curves reflecting selected ichnotaxa abundance against the background of geochemical and gamma ray records from the Mochras borehole. Contains British Geological Survey materials

Fig. 4
figure 4

Seven main lithofacies (1–7) and ichnofabrics in the Mochras profile; to the right—main types of bigradational contourite drift sedimentary cycles built of main lithofacies—full cycle, top cut-out cycle, bottom cut-out cycle (referred to Stow et al. 2002; Stow and Faugeres 2008). Contourite cycle divisions reflect variations in current velocity. C1–C5—“Full” contourite cycle. For lithology and trace fossil abbreviation see Fig. 2a, b

Fig. 5
figure 5

Sections of the Mochras core showing common types of lithology, ichnology, and cyclicity of sedimentation. a 775.79–776.81 m—bigradational (symmetrical) cycle, with gradually coarsening upward (cu) and gradually fining-upward (fu) phases, cut from the top by a conspicuous erosional surface (red bar), capped by sandstone and fining-upward succession. b 772.03–772.97 m—full bigradational (symmetrical) cycle, with gradually coarsening upward (cu) and gradually fining-upward (fu) phases. A and B—no primary sedimentary structures are visible, due to pervasive bioturbation. c, d 754.86–756.82 m—bigradational, top-cut cycle, some current ripples can be seen in the middle part; the cycle is followed by mudstone, in places with lamination, passing into the coarsening-upward phase of the next cycle. Characteristic bigradational (“symmetrical”) cycles with bioturbated coarser parts result from variations in current velocity and are referred to as contourite cycles (Stow et al. 2002; Stow and Faugeres 2008—see also Fig. 4)

Fig. 6
figure 6

Some sedimentary features of the investigated section. a Rip-up clasts pointing to the high energy of currents and erosion, 746.7 m. b Conglomerate resting on an erosional surface. Most pebbles are composed of local material (in some Phycosiphon incertum morphotype 3, Ph3, is visible), usually cemented by calcium carbonate. Shell debris is also common, 627.4 m. c Conglomerate—pebbles are composed of both local and exotic material (e.g., a larger chert pebble in the bottom-left), 627.3 m. d Erosional surface with coarse sandstone/grainstone, 630.9 m. e Mudstone–wackestone with erosional (channelized) surface and cross-bedded sandstone above. S, Schaubcylindrichnus; Ph2, Phycosiphon incertum morphotype 2, 683.3 m

Fig. 7
figure 7

Sedimentological and ichnological features. a Fluctuations of bottom currents: lamination (lam.) in the bottom (stagnation, no trace fossils), followed by altered periods of more stagnant/more dynamic (arrowed) current conditions. Changing oxygenation and nutrient delivery is reflected in different morphotypes of Phycosiphon incertum (Ph2, Ph3): small Ph3 (more stagnant conditions) and larger Ph2 (more dynamic conditions), depth 824.0 m. b Enlarged framed fragment of A. c Ichnofabric showing succession (tiering) of the earliest/shallowest Phycosiphon incertum morphotype 2 (Ph2), followed by Thalassinoides (Th) and bivalve burrow (b.b.), 603.6 m. d Cross-bedded (low-angle), very fine sandstone with mudstone intercalations. Bivalve resting trace ?Lockeia (b.b.), 818.67 m. e Thalassinoides burrow systems (Th) in siltstone/sandstone, in places infilled by Phycosiphon incertum morphotype 4 (Ph4)—arrowed. Numerous Phycosiphon incertum morphotype 2 (Ph2) in the background, 823.35 m. f Thalassinoides galleries of tubular burrows (Th, deeper tier), Phycosiphon incertum morphotype 2 (Ph2, shallower tier) in the background, 726.1 m. (g) Trichichnus—pyritized horizontal and oblique, unbranched or branched, thread-like, cylindrical structure produced by bacteria consortium, 712.6 m. h Planolites (Pl—thinner, randomly oriented tubes) and Thalassinoides (Th—larger burrows), 782.7 m. i Post-depositional colonization of a sandstone/grainstone bed with Phycosiphon incertum morphotype 2 (Ph2, earlier stage—shallower tier) and Thalassinoides isp. (Th, later stage—deeper tier), 821.6 m. j Teichichnus (Te, shallow tier) and pyritized, aggregated Trichichnus cylindrical vertical structures (Tr, deeper tier), 850.5 m. k Chondrites (Ch, deep tier) and shallow tier Thalassinoides (Th) and Planolites (Pl), 669.5 m. Scale = 1 cm

Fig. 8
figure 8

Other ichnological features. a Phycosiphon incertum morphotype 2—spreiten burrows visible in horizontal section. This level contains isorenieratane (Xu et al. 2018a, b); in that case, this biomarker does not indicate anoxic bottom conditions, 811.60 m. b Palaeophycus—cylindrical, pyritized structure with conspicuous lining, 785.2 m. c Schaubcylindrichnus (S)—cylindrical, lined structures flattened by compaction, 702.5 m. d Mottled mudstone (“undulated bedding”), sediment homogenized by meiofauna activity. This level contains isorenieratane (Xu et al. 2018a, b); in that case, this biomarker does not indicate permanent anoxic bottom conditions, 819.2 m. e Zoophycos galleries (Z) composed of fan-like, spreiten structures, 782.6 m. f Rhizocorallium (Rh), Thalassinoides (Th) and Phycosiphon incertum morphotypes 1 (Ph1) and 3 (Ph3), 801.8 m. g Chondrites, 803.1 m. h Lamination in claystone/mudstone, in places broken by bioturbation (Phycosiphon incertum morphotype 2, Ph2). In the lower and upper part “pinstripe” broken lamination with silty lamina and incipient ripples indicate recurrent activity of weak currents. Despite the presence of isorenieratane in this interval (Xu et al. 2018a, b), current activity and the presence of bioturbation do not support for anoxic conditions on the bottom surface. Depth 811.60 m. i Stiff ground and uneven omission surface with Thalassinoides (Th) and Skolithos (Sk); Phycosiphon incertum morphotypes 2 (Ph2) and 3 (Ph3) were left in soft sediment. Depth 730.0 m. Scale = 1 cm

Geochemical data taken from Xu et al. (2018a, supplementary data sheet) and gamma ray data from Woodland (1971) were subjected to analogous smoothing, with smoothing factor values in the 4–6 range.

The runs test was conducted to confirm statistically that the trace fossils, undulated bedding and lamination occur in a non-random order. This non-parametric test evaluates the null hypothesis of the two states being distributed randomly in a binary time series (Hammer and Harper 2006). For the present study, the adopted hypotheses are:

H0: distribution of occurrences is random;

H1: distribution of occurrences is non-random.

Runs are defined as uninterrupted sequences of one state, in the present case either an occurrence (N1) or non-occurrence (N2). The null hypothesis can be rejected if the observed number of runs (R) and the number of runs expected for the random time series (\(\bar{R}\)) are statistically different. PAST 4.0 software (Hammer et al. 2001) was used to compute the test statistic (Z-score). The null hypothesis was rejected at a significance level of 5% if an absolute value of computed Z-score was greater than the critical value of 1.96 taken from a standard normal table (Additional file 2).

A time series, both binary and continuous, can be seen as a sum of component constant-frequency waves. Mathematical transform operations serve as tools to extract the frequencies of component waves from time series. A range of well-established numerical methods that derive from the Fourier transform are applied in the spectral analysis of continuous time series. However, they are not suited for analysis of binary time series (Weedon 2003). Instead, the Walsh transform provides a way to conduct power-spectral analysis of binary data. The Walsh transform outputs the decomposed signal as a function of sequency, which is half of the average number of zero-crossings per unit of space (or time). From the point of view of results interpretation, the Walsh sequency can be equated with commonly recognized concept of frequency. Conceptual basis of the Walsh method and its applicability in cyclostratigraphy have been described by Negi and Tiwari (1984), Weedon (1989), van Echelpoel (1994) and Maiti and Tiwari (2012). According to Tiwari (1987) and Negi et al. (1993) the Walsh analysis not only outputs the peaks related to main orbital periodicities, but their component periodicities and beat periodicities as well.

Binary time series were subjected to analysis using the Walsh transform module implemented in PAST 4.0 software (Hammer et al. 2001). To find the optimal balance between the signal-to-noise ratio and spectral resolution, terminations of binary data series were adjusted by trial and error. Obtained power-spectra display peaks in the stratigraphic thickness domain (number of cycles per one metre, Fig. 9). Conversion of peak values from the thickness domain into the time domain was based on two alternative assumed average sedimentation rate value of 4.6 cm/kyr (upper number in the Fig. 9) and 4.1 cm/kyr (lower number in the Fig. 9). Consequently, time values corresponding to peak values were calculated as a proportion of 21,700 and 24,400 yr per one metre, respectively.

Fig. 9
figure 9

Overlapping Walsh power-spectra of the time series based on: a Phycosiphon morphotypes Ph2 and Ph3. b Thalassinoides and Schaubcylindrichnus. c Planolites and Trichichnus. d Undulated bedding and lamination. Spectra are scaled true to analysis results (not rescaled). Calculated time values assigned to peaks and upper abscissa axes based on two alternative sedimentation rates 4.6 cm/kyr (upper number) and 4.1 cm/kyr (lower number)

The most reliable sedimentation rates can be obtained for the Tenuicostatum Zone (~ Polymorphum Zone), where existing radioisotopic (Sell et al. 2014) and astrochronological (Huang and Hesselbo 2014; Rocha et al. 2016; Huang 2018) constraints give duration of ~ 0.8 Myr (although, assuming inferred hiatuses, this duration could be extended to ~ 1 Myr—Ruebsam and Al-Husseini 2020). With 33.8 m of sediments representing this time in Mochras, an average sedimentation rate of 4.1 cm/kyr is calculated. On the other hand, the sedimentation rate in the Pliensbachian section in Mochras (representing a similar depositional setting and stable sedimentation rate) was estimated at about 4.6 cm/kyr (Ruhl et al. 2016; Pieńkowski et al. 2021). Therefore, alternative time values were calculated for peaks in the spectra assuming this value. For comparison, the conversion was also tested with 3.3 and 2.8 cm/kyr sedimentation rates, which could be representative for more condensed sections with less stable sedimentation or more frequent erosional surfaces. However, the results based on these two lower alternative sedimentation rates were discarded as less meaningful.

3 Results

3.1 Lithology and sedimentary structures

The succession primarily comprises siliciclastic and calcareous deposits: mudstone/micrite, siltstone/wackestone and sandstone/grainstone with varying proportions of calcareous and siliceous grains of silt to fine-sand size. The siliciclastic input in coarser silty-sandy lithofacies is more pronounced in the lower part of the section with a CaCO3 content typically between 10 and 30%, while the mudstone contains a greater siliciclastic component and locally (805–840 mbs) is carbonate-free (Fig. 2a; Xu et al. 2018a). Macrofossils are mainly represented by pelagic cephalopods (Xu et al. 2018a; Ullmann et al. 2021). Crinoids are much less common in the Toarcian than in the Pliensbachian section (Pieńkowski et al. 2021) and are most likely redeposited from shallower settings due to their buoyancy (Savarese et al. 1997). Benthic shelly organisms are rare, and they are represented by in situ burrowing bivalves found in a few horizons (Fig. 7c, d). Benthic foraminifers are also present, and their assemblages are important to determine bottom life conditions (Copestake and Johnson 2014; Reolid et al. 2019). Drifted flora occurs locally, mostly in the Tenuicostatum Zone and the Exaratum Subzone, with some occurrences also in the Variabilis Zone. Low-angle cross-lamination is common, particularly in the Exaratum, mid-Bifrons, mid-Variabilis, Thouarsense, and mid-Pseudoradiosa zones. Siderite (usually dispersed, in places forming bands and nodules, oxidised to iron oxide) is common in the Tenuicostatum Zone and Exaratum Subzone, as noted by Xu et al. (2018a). Dewatering/compactional cracks are observed only in a few horizons (in the Bifrons and Variabilis zones).

As in the Pliensbachian section, the same six main lithofacies types (Pieńkowski et al. 2021; their Fig. 8) are identified in the Toarcian section of Mochras: (1) Dark grey, planar-laminated siliceous or calcareous micritic mudstone and siliceous claystone with continuous (without bioturbation) and broken lamination (with few bioturbation structures); (2) Massive, poorly bedded mudstone/carbonaceous micritic mudstone, in some cases showing centimetre- to decimetre-scale banding with faint, slightly undulated (undulated bedding), or cuboid parting, which is connected to homogenization by meiofaunal bioturbation of sub-millimetre size (Figs. 4, 5 and 8d); discrete trace fossils are relatively rare; (3) Pinstripe carbonaceous mudstone that is laminated with wackestone-siltstone, in which planar laminae are often discontinuous and lenticular to wavy, planar parallel, or low angle cross-lamination, starved ripples are present, and bioturbation structures can obliterate these in mottled mudstone-wackestone (Figs. 4 and 5); (4) Interlaminated, heterolithic siltstone or wackestone with mudstone, usually bioturbated, with only local sedimentary structures; this lithofacies shows irregular arrangement of mudstone, wackestone-siltstone or grainstone-sandy siltstone in pockets, lenses and streaks, and less commonly, a rapid alternation of thin irregular layers of these three lithologies (Figs. 4 and 5); (5) Packstone/siltstone to very fine sandstone, with primary sedimentary structures—usually obliterated by bioturbation—including parallel bedding and small-scale cross-bedding (Figs. 4 and 5); (6) Sandstone-grainstone, usually bioturbated, with primary structures (parallel bedding and cross-bedding) observed in some horizons (Fig. 4). Additionally, the conglomerate lithofacies (7) is present—it is too rare to play an important role in basic models of cycles, but the presence of conglomerate beds (composed both of local and exotic material—Fig. 6a–c) is important as an indicator of intermittent high-energy currents. Also of note (compared to Pliensbachian) is a general rarity of lamination.

Contacts between these different lithofacies may be either gradational, yielding indistinct bedding (Figs. 4 and 5), or sharper due to the erosive action of relatively strong bottom currents, thereby yielding more distinct bedding (Figs. 46). Compared to the Pliensbachian section (Pieńkowski et al. 2021) of note are much more common sharp, erosional boundaries in the Mochras Toarcian section, associated with coarse-grained sediments (including conglomerates, either with clay pebbles derived from local compacted mudstone or of exotic lithology such as limestone and chert), pointing to high-velocity currents, erosion/non-sedimentation and hiatuses (Fig. 6). In places, hiatal levels are marked by induration of the sediment surface occurring as semi-consolidated/firm muddy/silty substrates with sharp-walled, unlined, uncompacted passively filled burrows (Fig. 7i).

Lithofacies types 1–7 and related ichnofabric in the Toarcian of Mochras are not arranged randomly, but appear in a cyclical order (Figs. 2a, b, 4 and 5). Fully developed, usually 0.5–3 m thick, bigradational grading sequences (some ‘couplets’ of previous authors) commonly begin from mudstone lithofacies type 2 (representing mixed layer, mottled by burrowing activity of the meiofauna), or mudstone lithofacies type 1 (laminated), gradually passing into lithofacies type 3—pinstripe mudstones with ripple-drift cross laminated siltstones, or lithofacies 4—bioturbated mudstone-wackestone, followed usually by bioturbated wackestone-siltstone and packstone-grainstone with traces of stronger currents (Fig. 5). Above this coarsening-upward part is a fining-upward suite, developed in reverse order, although less complete; usually only lithofacies types 6–5–4 are present. This fining-upward phase of the sequence is usually thinner than the coarsening-upward phase. In some cases, the whole sequence is incomplete; either top or bottom parts can be missing (top cut-out cycles or bottom cut-out cycles—Fig. 5). These successions are like those observed in the Pliensbachian part of the section (Pieńkowski et al. 2021). The main difference is the more common occurrences of erosion surfaces in the Toarcian section, locally with lithofacies type 7 and hiatuses (Fig. 6). Of note are also the markedly finer sediments occurring of earliest Toarcian age (Tenuicostatum Zone, and ealiest Exaratum Subzone) and again in the Thouarsense-Dispansum zonal transition (Figs. 2, 3).

3.2 Trace fossils, ichnofabrics

Trace fossils exhibit sharp outlines and possess a characteristic geometry that allows their classification in terms of ichnotaxonomy, while bioturbation structures have less distinct outlines and do not display a recurrent geometry (Wetzel and Uchman 2012). Below, trace fossils are briefly described. The trace fossils show significant similarities to those described in Pieńkowski et al. (2021) from the Pliensbachian. Their synopsis is presented in Table 1.

Table 1 Trace fossils of the Toarcian deposits in the Mochras core

Phycosiphon incertum with its four morphotypes (Ph1, Ph2, Ph3, Ph4) is the most abundant trace fossil in the Mochras Toarcian. The exception is the Tenuicostatum Zone (and to a lesser extent also the Thouarsense-Dispansum zonal transition), characterised by Phycosiphon scarcity and dominance of Trichichnus. Previous ichnological analysis of contourites has revealed changes in ichnofabric attributes over relatively short lateral distances, including trace fossil composition, cross-cutting relationships, or ichnofabric index (Hüneke and Stow 2008; Wetzel et al. 2008; Wetzel and Uchman 2012; Rodríguez-Tovar et al. 2016, 2017; Rodriguez-Tovar and Molina 2018). Recently, Dorador et al. (2019) and Pieńkowski et al. (2021) have indicated that the distance to the bottom current ‘core’ exerts a tangible and widespread influence on specific macro-benthic tracemaker communities in contourite deposits. This parameter itself reflects other bottom current features, such as hydrodynamic energy, grain size, nutrient transport, etc. Specifically, strong domination of Phycosiphon is unique for the Pliensbachian-Toarcian contourite deposits in Mochras. Phycosiphon, common in turbidites and hyperpycnites (e.g., Wetzel and Uchman 2012; Knaust et al. 2014), is also reported from contourite deposits (Baldwin and McCave 1999; Wetzel et al. 2008) but as a rare, subordinate trace fossil. Lack of Phycosiphon in other contourite deposits may be more apparent than real: in previous papers diffuse Phycosiphon burrows may simply have been described as “bioturbation”. In particular, this ichnotaxon could be difficult to recognize in recent unconsolidated contourite deposits. Other deposit-feeding feeding traces, e.g., Rhizocorallium, are rare.

Schaubcylindrichnus isp. is common through the late Bifrons and Variabilis zone strata. In the Toarcian of Mochras, Teichichnus isp. is much less numerous than in the Pliensbachian (Pieńkowski et al. 2021). Teichichnus and Schaubcylindrichnus do not usually occur together. Thalassinoides crosscuts Phycosiphon of the morphotypes Ph1, Ph2 and Ph3, but is cut by Ph4, Schaubcylindrichnus and Teichichnus. Trichichnus (relatively rare in Pliensbachian in Mochras, see Pieńkowski et al. 2021) is particularly common in the Tenuicostatum Zone, which is associated with declining abundance in Phycosiphon and low ichnodiversity. Trichichnus is also abundant in the mid-Falciferum Subzone, in the Thouarsense-Dispansum zone transition, and (less abundantly) in the mid-Pseudoradiosa Zone. The presence of Trichichnus usually points to oxygen-depleted conditions, but interpretation must be compatible with occurrences of other trace fossils, representing shallower tiers and/or more oxygenated conditions. The abundance and diversity of other trace fossils disaffirms dysoxic conditions in this case.

Among other trace fossils, of note are occurrences of dwelling structures of filter-feeding organisms, building vertical structures stabilized by a mucus lining (Skolithos, ?Monocraterion, Arenicolites, Siphonichnus, cf. Polykladichnus). With respect to behavioural groups, it should be noted that the general abundance of suspension feeders in the Toarcian section of Mochras, although more frequent than in the Pliensbachian (Pieńkowski et al. 2021) is still low, probably because their filter apparatus can easily be plugged when the mineral suspension concentration is high (Pieńkowski 1985; Thistle et al. 1991).

All described trace fossils are important components of ichnofabrics (ichnofabric is understood as an overall texture and structure formed by bioturbation or bioerosion; see Bromley and Ekdale 1986). Constituents of ichnofabrics may be attributed to several controls and attributes, in particular to the tiering patterns and the manner of colonization (Ausich and Bottjer 1982; Taylor et al. 2003). The complete (“ideal’) tiering pattern in the Mochras Toarcian section can be summarized as follows. The shallowest tier consists first of an indistinct mottling that is produced in the mixed layer by bioturbation in water-saturated soupy sediment near the sea floor, followed next by the shallow tier of Phycosiphon (Ph1, Ph2, Ph3) showing a patchy distribution. The middle tier is occupied by Thalassinoides, and slightly deeper Schaubcylindrichnus, Planolites, rare Teichichnus, and yet deeper another generation of Phycosiphon (Ph4), reworked Thalassinoides and rarely Teichichnus. The yet deeper tier is characterized by Teichichnus and Zoophycos, and the deepest one is occupied by Trichichnus. Rare traces (cf. Polykladichnus, Skolithos, Arenicolites, Siphonichnus, Rhizocorallium) usually occupy a middle tier, below Phycosiphon (Ph1, Ph2, Ph3). On the other hand, Chondrites occupies a deep tier. However, such a complete tiering pattern occurs rarely, and the most common tiering pattern is limited to shallowest tiers: Phycosiphon (Ph1, Ph2, Ph3), Thalassinoides (middle tier), with Schaubcylindrichnus slightly deeper, and Trichichnus occurring at the deepest level.

4 Discussion

4.1 Sedimentary environment, ichnodiversity, cyclicity of sedimentation

4.1.1 Sedimentary environment

The lithofacies in the Toarcian at Mochras contrast with those in other settings for the UK Jurassic (as is also the case for the Pliensbachian). For example, the Toarcian in the Cleveland Basin, some 300 km to the NE of Cardigan Bay, is represented by organic-rich (black) shale up to late Bifrons Zone age, known as the Mulgrave Shale (formerly the Jet Rock), similar to the Schistes Cartons of France and Posidonia Shale of Germany (Jenkyns 1988; Atkinson et al. 2022). These strata were deposited in a restricted shelf basin (Powell 2010) that was episodically affected by storm processes (e.g., Kemp et al. 2018). Higher levels within the Cleveland Basin Toarcian are clearly deposited in a storm-dominated shoreface setting (e.g., Hesselbo and King 2019).

Based on sediment transport processes, the action of two different types of deep-water currents and their depositional products have been recognized in the Mochras section, namely contourite drift facies produced by along-slope contour currents (see Pieńkowski et al. 2021 for the Pliensbachian part of the section) and turbidite deposits created predominantly by downslope sediment density flows (Xu et al. 2018a—Exaratum Subzone, Serpentinum Zone, of the Toarcian section). Xu et al. (2018a) illustrated few cm-scale sharp-based beds in the T-OAE interval described as turbitites. However, fining upwards of these beds is not gradual, with coarser intercalations sharply separated not only from below, but also from above without continuous transition; therefore, these beds may alternatively represent repeated intensifications of along-slope currents. It is also possible that sediments of distal turbidites could occasionally have been “pirated” by contourite currents (Gong et al. 2017; Rodríguez-Tovar et al. 2019a, b), and such a situation could also have occurred in Mochras.

Apart from this T-OAE interval, the Toarcian section in Mochras does not reveal any features of typical turbidites. The silty-sandy beds and laminae in the studied section show no systematic vertical grading or stacking of structures, such as those recognized in the Bouma (1962) intervals. The Mochras section overall records continuous sedimentation with structures characteristic of current traction, controlled by fluctuation of their speed; therefore, deposition by turbiditity currents was unlikely. In addition, turbidites are primarily bioturbated from the top (e.g., Uchman and Wetzel 2011) and the uppermost layers of a turbidite exhibit total bioturbation, which decreases with depth as lamination and cross bedding appear. In contrast, contourites typically exhibit more continuous and uniform bioturbation, which appears throughout the entire contourite bed; this situation is typical for both the Pliensbachian and Toarcian of Mochras. However, the variability of ichnofabric pattern in contourites may depart from this typical picture (Rodríguez-Tovar et al. 2019a, b, 2022).

Interpretation of a wide spectrum of bathymetry and occurrences of contourite currents is recognized, i.e. everywhere the sea floor had a considerable relief, commonly of more than a hundred metres amplitude, comprising moats, drifts, mounds and channels (e.g., Bein and Weiler 1976; Surlyk and Lykke-Andersen 2007; Esmerode et al. 2008; Van Rooij et al. 2010; Rebesco et al. 2013), although the definition of Faugeres and Stow (2008) is also applied, according to which the term contourite should be generally used for sediment accumulated in relatively deep water (markedly deeper than c. 200 m-deep shelf platform) and deposited or significantly reworked by stable currents.

The deposits in Mochras can be classified as mixed siliciclastic-calcareous/biogenic contourites, the most frequent contourite facies in the modern oceans (Fagueres and Mulders 2011). In less bioturbated sections of Mochras, primary sedimentary structures are visible and are mostly represented by current structures such as planar parallel lamination, low angle cross-lamination, starved ripples, in places erosive bases, and gradational normal grading bed transitions (Fig. 5a), like cases studied by Shanmugam (2000) and Knapp et al. (2017). Stagnant conditions or very slow currents allowed vertical settling of the suspended particles from the nepheloid layer (Ewing and Thorndike 1965) producing mottled mudstone (Fig. 8d), or laminated mudstone-claystone (Figs. 7a and 8h), where the grain-supported laminae are interpreted to be the result of very weak contour currents that winnowed out clay-sized sediment (e.g., Shanmugam 2000).

Higher bottom current velocities led to deposition of silt or sand layers with planar or low-angle cross-bedding, produced by bedload transport (Figs. 5c, d and 7d) and periodically these currents were strong enough to erode older sediments and carry coarse-grained sediments, including clay intraclasts and pebbles (Fig. 6). Compared to the Pliensbachian (Pieńkowski et al. 2021), visible plant remains are more frequent (see also Ullmann et al. 2021) and indicate more phytodetrital pulses in connection with fluvial discharges in the hinterland, which possibly could have led to acidification (Müller et al. 2020) and intermittently reduced salinity of sea water (Dera and Donnadieu 2012), both adding (along with the Toarcian rise in pCO2; see Hermoso et al. 2009) to collapses of the carbonate factory (Bodin et al. 2023), suggested particularly for the Tenuicostatum Zone to Exaratum Subzone interval (Fig. 2a). Relatively weaker bioturbation can be attributed to oxygen depletion or high sedimentation rate (Stow and Fagueres 2008); assuming rather stable sedimentation rate, weaker bioturbation is here largely attributed to decreasing circulation and oxygenation. One should bear in mind that strong bottom currents (Fig. 6) also do not favour preservation of biogenic structures (Tucholke et al. 1985).

Fully developed bigradational intervals in the Mochras section (Fig. 5) correspond to a standard contourite sequence (Faugeres et al. 1984; Stow and Holbrook 1984; Stow et al. 2002; Huneke and Stow 2008; Rodríguez-Tovar and Hernandez-Molina 2018), a continuation of the situation for the Pliensbachian (Pieńkowski et al. 2021). The standard contourite model was enhanced by introduction of interval divisions (C1 to C5; Stow et al. 2002; Stow and Faugères 2008, their Fig. 13.9) and recognition of variations within partial contourite sequences (Rebesco et al. 2014; Shanmugam 2017), corresponding to observations in the Mochras profile (Figs. 4 and 5) where most frequent vertical sequences consist of lithofacies types 1/2–3–4–5–6 (in coarsening-upward order) and subsequent fining-upward couplet composed of 5–4 lithofacies types. The fining-upward phase of the fully developed cycle is usually thinner and incomplete. In places, these sequences are interrupted by erosional surfaces and coarse sediments, including conglomerates (lithofacies type 7; Fig. 6). Periods of strong currents marked by erosional surfaces and coarser material, pointing to intermittent non-deposition/erosional periods, appear first in the late Falciferum Subzone of the Serpentinum Zone, and occur mostly in the late Bifrons Zone, Variabilis-Thouarsense transition, and the late Levesquei Subzone (Fig. 2a, b). In such cases the bigradational grading cycles are often incomplete, lacking their upper parts (top-cut-out cycles; see Fig. 5a). The depositional rate at Mochras during the Toarcian remained generally stable and relatively high, but the sections mentioned above show condensation (Figs. 2a, b, 5 and 6).

4.1.2 Ichnodiversity

Occurrences of trace fossils also show a cyclic character, more or less corresponding with lithological changes and sedimentary cycles. At one end of a spectrum is the domination of opportunistic, r-selected Phycosiphon, diffused shape of trace fossils, indications of soft-bottom conditions (Fig. 7f), mostly simple tiering, and scarcity of highly specialized K-selected forms, all indicating a generally stressful environment (e.g., Ekdale 1985), which was probably caused by high sedimentation rate associated with unstable substrate and intermittent benthic food availability, interrupted by transient oxygen-depleted, more stagnant conditions.

The other end of the spectrum is characterised by higher substrate stability (stiffer conditions), lower sedimentation rate, continuous delivery of suspended nutrients by currents (often adsorbed onto suspended clay minerals – Mayer 1994; Thistle et al. 1985), and usually fairly good oxygenation of the bottom, leading to more complex tiering under equilibria (Taylor et al. 2003). A high degree of bioturbation is characteristic for contourites due to additional food supply (the vertical particle flux is supplemented by lateral current-carried supply) and faunal abundance (Suess 1980; Wetzel et al. 2008). In agreement with Caswell and Frid (2017), changes in community composition are usually linked to local redox conditions, whereas changes in populations of r-selected opportunists are driven by primary productivity.

4.1.3 Cyclicity of sedimentation and spectral analysis of ichnological data

Four orders of cycle duration are determined based on visual inspection of the sedimentary succession. A 4th-order cyclicity is interpreted as a superposition of precession (c. 20 kyr) and obliquity terms (c. 38 kyr, Waltham 2015). Precession and obliquity cycles are hardly distinguishable from each other in direct observation, but are identified discretely by spectral analysis. The 3rd and 2nd order cycles are identified with short eccentricity (c. 100 kyr) and long eccentricity (c. 405 kyr), respectively (Kent et al. 2018). A 1st-order cyclicity is attributed to eccentricity modulation of the c. 2.4 Myr period, which is caused by secular resonance between Earth and Mars (Hinnov 2000; Laskar et al. 2004, 2011).

As described previously for the Pliensbachian (Pieńkowski et al. 2021), the distinction of cycles in the Mochras Toarcian is based mainly on grain size and ichnological features and their lower boundaries are placed in the fine-grained, mottled, or laminated mudstone (lithofacies 1 and 2; see Fig. 4). Both the fully developed (bigradational), and the incomplete lithological couplets with their ichnological content, constitute the 4th order cycles, basic ‘building blocks’ of the hierarchical order of cycles in Mochras (Figs. 2a, b and 5). Commonly, in these cycles, meiofauna mottling (undulated bedding) appears first, followed by colonization of Phycosiphon (Ph3) as the first recognizable trace fossil, then Ph2 and other trace fossils return. These 4th order cycles are arranged in higher hierarchical successions (3rd order cycles), usually containing four, locally five, 4th order cycles. As every 3rd order cycle commences with the 4th order cycle, the 3rd order cycles start from meiofauna mottling and Ph3. Schaubcylindrichnus, occasionally accompanied by Planolites, tends to appear at the tops of these cycles. In many cases Zoophycos can be found in the middle, and occasionally K-selected (equilibrichnia) forms, such as Skolithos, Arenicolites and Siphonichnus (dwelling/suspension feeders’ structures), bivalve resting tracks Lockeia, larger deposit feeder structures Rhizocorallium, and Chondrites (chemichnion), appear at boundaries of these cycles.

Next, 2nd order cycles are composed of four 3rd order cycles. Sixteen out of 24 of the 2nd order cycles show occurrences of K-selected (equilibrichnia) forms (such as Skolithos, Arenicolites, Monocraterion, Siphonichnus, Rhizocorallium and Chondrites at its boundaries, pointing to more stable conditions. Both 2nd and 3rd order cycle boundaries show sharp-walled, uncompacted, passively filled burrows that point to condensation/non-sedimentation (Fig. 8i).

Erosional surfaces, commonly associated with coarse-grained sediments, always indicate sedimentation breaks and play an important role in the interpretation of the boundaries of the cycles. Usually, boundaries of the 2nd or occasionally 3rd order cycle are placed at these surfaces. High velocity contourite currents are known to generate erosional surfaces and contourite lag facies, resulting from winnowing, reworking processes and hiatuses (Faugerez and Mulder 2011). In Mochras, the lag facies contain a large range of grain sizes and compositions, forming irregular centimetre-thick to metre-thick beds of poorly sorted sediments, mainly coarse sands, gravels, and pebbles or rip-up clasts (Fig. 6).

The highest, 1st order, cycles are composed of six 2nd order cycles. The beginning of the Toarcian sequence of these cycles is anchored at the Pliensbachian-Toarcian boundary, where the succession of four previous 1st order cycles of the latest Sinemurian to Pliensbachian age ends (Pieńkowski et al. 2021). These 1st order cycles in the Toarcian differ from those observed in the Pliensbachian, mainly because a regular, short- and long-term cyclicity of CaCO3 content (Ruhl et al. 2016; Pieńkowski et al. 2021) disappears in the Toarcian, making the CaCO3 content generally of less use in reconstructing the cyclicity. Reduced CaCO3 content can be attributed to environmental change that occurred at the beginning of the Toarcian. In a similar fashion to the Pliensbachian 1st order cycles, the greatest ichnodiversity tends to appear in their middle parts. Higher ichnodiversity usually also coincides with condensation (Fig. 2a, b). This interpreted ~ 2.4 Myr cyclicity retains its regularity throughout the entire Pliensbachian–Toarcian section of Mochras, although Charbonnier et al. (2023) postulated much shorter cyclicity (~ 1.6 Myr) for the Sancerre core in Paris Basin—see discussion below.

Because generally the whole Toarcian in Mochras is dominated by Phycosiphon, any lack of this morphotype is noteworthy, as it indicates an exceptionally unfavourable environment, most likely associated with stagnation and oxygen depletion. Of particular note is the fine-grained interval between 823 and 868 mbs, mostly Tenuicostatum Zone, dominated by mudstone and claystone, with significantly reduced CaCO3 content, frequent siderite occurrences (Xu et al. 2018a) and a marked foraminiferal biotic crisis (Reolid et al. 2019). Here, Phycosiphon is reduced in frequency, and other trace fossils are very scarce, while Trichichnus and undulated bedding are prolific. Only for the middle of this interval (~ 838 to ~ 853 mbs) can some intermittent improvement of bottom life conditions be inferred, indicated by less continuous occurrences of Trichichnus, brief re-appearances of Phycosiphon, and presence of cf. Polykladichnus and Chondrites (Fig. 2a). This slight recovery is also observed in foraminiferal assemblages (Reolid et al. 2019).

Less severe ichnological crises are noted in the Thouarsense-Dispansum zones with Trichichnus occurrences and diminished ichnodiversity. Here, Phycosiphon scarcity is also observed although, compared to the earliest Toarcian, less pronounced. Relatively abundant Trichichnus occurrences in the mid-Pseudoradiosa Zone are accompanied by appearances of other diversified trace fossils; thus, this section can be regarded as not affected by oxygen depletion. The Bifrons and Variabilis zones generally represent well-oxygenated intervals, shown by common occurrences of Thalassinoides, Schaubcylindrichnus, Teichichnus and other trace fossils, including domichnia; only over depths 710–720 mbs is a slight decrease in oxygenation inferred, marked by decreased ichnodiversity and presence of Trichichnus.

Smoothed curves of particular ichnotaxa occurrences juxtaposed with smoothed Zr/Rb, Ti/Al, Si/Al, Ca, TOC, δ13CTOC, and gamma ray records reveal a high degree of similarity (Fig. 3). The relationship of particular trace fossil occurrence curves to each proxy record is, to some extent, disparate, and the parallelism of respective curves varies through the succession. Nonetheless, the outlines of several curve crests are visually coherent, and the phase difference is insignificant given the timescale. Most notably, the T-OAE-negative CIE has coincident expressions in the records of all analyzed ichnotaxa. The Zr/Rb record, a proxy for silt/clay ratio, displays resemblance to the highest number of ichnotaxa occurrences (five; Fig. 3), and the Thalassinoides occurrence curve resembles the highest number of proxy records (five). The best visual coherence appears to occur between the curves of the following: Zr/Rb—Phycosiphon type 3, Thalassinoides and Planolites; Si/Al—undulated bedding; Ca content—Thalassinoides and undulated bedding; TOC content—Schaubcylindrichnus.

Interpretation of cyclicity and resulting calculation of time based on these cycles attributed to certain orbital cycles becomes more complex in sections with erosional surfaces, associated with inferred hiatuses in sedimentation, resulting in less regular sedimentation rate and cyclicity. It is reasonable to interpret that much of the lost time is represented at the erosional/hiatal surfaces. It is difficult to say how long lasting were time intervals of non-sedimentation, or how deep was erosion of previously deposited sediments. In such cases, the 1st order (~ 2.4 Myr) cycles can provide some time frameworks, allowing for approximate adjustment of the 2nd and 3rd order cycles to the ~ 2.4 Myr duration of 1st order cycles. This is particularly important in the Toarcian above the Serpentinum Zone, where most erosional/hiatal surfaces with coarser sediments occur, usually concentrated in the middle parts of the 1st order cycles (Fig. 2a, b).

For numerical cyclicity (spectral) analysis only frequently occurring trace fossils can be used. Phycosiphon incertum is by far the most common trace fossil in the Mochras section. As it is represented by four distinct morphotypes (Ph1, Ph2, Ph3, Ph4), they are treated separately for analysis of cyclicity (in the Toarcian Ph2 and Ph3 were used). Thalassinoides, Schaubcylindrichnus, Planolites, and Trichichnus are also frequent enough to be used for analysis of cyclicity as well. Other ichnotaxa are not frequent enough for spectral analysis, although their cyclic appearances yield useful information concerning a more general hierarchy of cycles. Additionally, lamination and undulated bedding can be used for spectral analysis.

The runs test results indicate the non-random distribution of either trace fossil, lamination and undulated bedding occurrences in the studied sedimentary succession (Additional file 2), demonstrating the advisability of time series analysis. Absolute values of all computed Z-scores are within the critical region of > 1.96, which allows rejection of the null hypothesis of the data randomness at the 95% confidence level.

The peak values of the Walsh power-spectra (Fig. 9) correlate to the calculation-based expected duration of the orbitally forced periodicities (Table 2). The resultant spectral peaks (Fig. 9) have estimated time values assigned as explained in Sect. 2. Component frequencies of precession, obliquity and short eccentricity, as well as beat frequencies, are present in the resultant spectra besides the main terms (Tiwari 1987; Negi et al. 1993). The longer the periodicity, the more prone it is to be blurred by instantaneous changes in the sedimentation rate. Besides, shorter-term periodicities can be recorded with higher precision than these longer-term, as a larger number of them are contained within the studied time series.

Table 2 Spectral peak values recognized in the present study against the background of calculated present-day component orbital periodicities

Either one or more spectral peaks in the sequency range of circa 0.9–1.2 cycles/m are reflected by most of the analysed time series, with the exception of Trichichnus (Table 2). These prevalent, albeit relatively low-power peaks are interpreted as precessional terms corresponding to the visually determined 4th order cycles. Schaubcylindrichnus occurrences reflect the most (three) component terms in the range of precession, which in this case are characterised by spectral power on par to the other periodicities.

Periodicities in the range of either main obliquity term (Phycosiphon Ph3 and undulated bedding) calculated for the Toarcian (Waltham 2015), or theoretical component obliquity terms (remaining time series) are distinct in all Walsh spectra, even though obliquity is not readily distinguished visually in the sedimentary succession. Periodicity of c. 28 kyr is observed in the occurrences of Phycosiphon (Ph2 and Ph3) and Planolites, which can probably be identified with the shortest component term of obliquity (Tiwari 1987; Laskar et al. 2011). Presumably, the peak near the 0.31 cycles/m value displayed by Schaubcylindrichnus occurrences (Fig. 9) can be associated with the combined effect of obliquity and precession, as proposed by Berger (1977).

Peaks interpreted as short eccentricity (3rd order) terms are the most intensely (high-power) represented in the spectrograms (Fig. 9). Periodicities in the range of c. 105–120 kyr recorded by Phycosiphon Ph2 and Ph3, Schaubcylindrichnus and Planolites (Fig. 9) can be identified as the longer component term (c. 112 kyr) of short eccentricity cycles, presumably reflecting the 3rd order cycles distinguished in the sedimentary and ichnological record (Fig. 2a, b). In addition, ~ 90 kyr periodicity displayed by Thalassinoides can be interpreted as the shortest eccentricity component term (~ 95 kyr).

Trichichnus occurrences record also the periodicity of the ~ 170 kyr term, which likely can reflect a cyclicity of c. 150–170 kyr, also present in the Pliensbachian of Mochras, and there preliminarly regarded as non-astronomical (Pieńkowski et al. 2021). However, this cyclicity could be associated with the amplitude modulation (AM) of the obliquity (Hinnov 2000). Such AM 173 kyr periodicity was hitherto detected in the sedimentary record by the means of time series analysis by Laurin et al. (2015), Boulila et al. (2018), Vahlenkamp et al. (2018) and Huang et al. (2021).

The peak clusters present in the leftmost parts of the Planolites, Thalassinoides and Trichichnus spectrograms may be associated with 2nd order ~ 405 kyr cycles (registered by recurrent appearances of Skolithos, Siphonichnus, Arenicolites, and Chondrites, which were not included in the spectral analysis due to their low frequency). However, this remains a presumption due to spectral resolution of spectra below the sequency values of 0.1 cycles/m being inadequate to confidently interpret these peaks in the time domain.

The most complete representation of orbital periodicities is reflected by both Phycosiphon morphotypes, Thalassinoides, Schaubcylindrichnus and Planolites (Fig. 9) which clearly display the frequencies of the short eccentricity terms (~ 95 and 112 kyr), obliquity terms (~ 29 and ~ 40 kyr) and precessional terms (~ 17–25 kyr). Spectra of both Phycosiphon morphotypes show a good congruence, especially considering an interpreted longer component term of short eccentricity. In the case of Phycosiphon Ph2, it can be supposed that evident periodicities associated with either 4th or 3rd order cycles are derivative of periodic short non-occurrences of this pervasively occurring trace fossil. Interestingly, Trichichnus occurrences seem solely to reflect periodicities related to obliquity and its AM frequency.

Longer-term ‘grand cycles’ which are not included in spectral analysis, are characterized by a gradually growing and subsequently falling Phycosiphon frequency and general ichnodiversity. Judging by the duration of ~ 100 kyr and ~ 405 kyr eccentricity cycles, these longer-term eccentricity “grand cycles” would be of duration c. 2.4 Myr, corresponding to the period of around 2.4 Myr eccentricity modulation related with Mesozoic greenhouse sequences, caused by Earth–Mars secular resonance (Hinnov 2000; Laskar et al. 2004, 2011; Martinez and Dera 2015). Whereas individual repetitions of the 2.4-Myr cycle range from 2.0 to 2.9 Myr in the Cenozoic (Pälike et al. 2006), cyclostratigraphic studies suggest that this period fluctuated between maximal values ranging from 1.3–3.3 Myr during the Mesozoic (Ikeda and Tada 2013, 2020). Indeed, the periodicity of grand eccentricity cycles prior to 50 Ma remains uncertain. Charbonier et al. (2023) postulated a chaotic behaviour of the Solar System and ~ 1.6 Myr Earth-Mars resonance cyclicity (instead of ~ 2.4 Myr) for the Early Jurassic, based on the Sancerre-Couy core (Paris Basin). However, Wu et al (2022), while also suggesting transient chaotic behaviour of the Solar System in the Late Cretaceous of the Songliao Basin (China), revealed evidence for two chaotic secular resonance transitions in the orbital motions of Earth and Mars, involving the orbital eccentricity modulations of 2.4 Myr and 1.2 Myr cycling, without indication of ~ 1.6 Myr cycling. We support the ~ 2.4 beat (Martinez and Dera 2015), because the observed general ichnological cycles, which are built on well-established hierarchy of sub-cycles, are much less compatible with the ~ 1.6 Myr duration of grand orbital cycles of Charbonier et al. (2023) than with the ~ 2.4 Myr cycles (Fig. 2a, b).

The existence of yet longer cyclicity, ~ 7–8 Myr eccentricity (Martinez and Dera 2015; Pieńkowski et al. 2021; Charbonier et al. 2023) is supported, based on two possible such cycles in the combined Pliensbachian-Toarcian section.

4.2 Toarcian deep sea current circulation

Oxygen deficiency and resulting decline in ichnodiversity can be attributed to the diminishing current intensity, but also to other related or independent causes, including high organic productivity and sea level. The earliest Toarcian crisis that had begun already in the latest Pliensbachian was exceptional in its severity (Xu et al. 2018a; Reolid et al. 2019; Pieńkowski et al. 2021; Bodin et al. 2023). Most likely, the benthic crisis was associated with severe stagnation of circulation through the Laurasian Seaway (and the Cardigan Bay Strait) at that time (Van Schootbrugge et al. 2019). Thermohaline circulation driven by density differences between water masses due to variations in water temperature and salinity is still regarded as the major driving force of contourites (McCave 2008; Faugeres and Mulder 2011; Rebesco et al. 2014). The biogenic input from shallower zones, likely carbonate platforms, was less marked than in Pliensbachian, and was replaced by a stronger siliciclastic input, which also carried more floral debris and possibly connected to acidification (Xu et al. 2018a, b; Müller et al. 2020)—although δ44/40Ca and δ88/86Sr records do not support acidification (Li et al. 2021). Bottom currents carried in suspension a considerable amount of particulate organic matter, supplying food to deep-marine benthic organisms (Thistle et al. 1985). As for the Pliensbachian, it is proposed here that the deep-water circulation in the elongated, NE-SW trending Cardigan Basin (Fig. 1) was forced by enhanced bottom-water circulation, i.e. cooler and denser waters flowing with changing velocity from the Boreal Sea, around the Shetland Platform-Scottish Landmass island, towards the south, to the Peri-Tethys/proto-Atlantic, approximately parallel to the bathymetric contours of the margin of the Welsh Platform (Fig. 1).

Interpretations of circulation (Bjerrum et al. 2001; Dera et al. 2009; Dera and Donnadieu 2012; Ruvalcaba Baroni et al. 2018) point to the predominant southward flow from the Arctic into the Tethys through the Laurasian Seaway during the Early Jurassic. The Cardigan Bay Strait, linking cooler and shallower waters of the Boreal Sea with warmer and deeper waters of Peri-Tethys (Fig. 1), would then have been situated to sustain a continuous (persisting for up to millions of years and over large areas—Shanmugam 2008, 2017) thermohaline-driven contour current circulation between these two marine realms (Pieńkowski et al. 2021). These invigorated flow conditions were punctuated by times of sluggish circulation or stagnanation characterised by loss of ichnodiversity and Phycosiphon crises (usually also by mass Trichichnus occurrences), as well as appearance of undulated bedding. These intervals occur in the Tenuicostatum Zone through to earliest Exaratum Subzone age strata and, to a lesser extent, in the latest Thouarsense Zone to Dispansum Zone.

This sluggish circulation is associated with climate warming under high atmospheric pCO2, when the Laurasian Seaway was influenced by strong clockwise circulation in the Tethys, which brought warm saline waters onto European shelves and then diminished the effects of flow from the north through the Laurasian Seaway (Ruvalcaba-Baroni et al. 2018). In the same time, loss of polar sea ice and stronger high-latitude continental runoff rates could further result in a weakening of thermohaline circulation, and bottom water oxygen depletion also in deep marine settings (Dera and Donnadieu 2012; Van de Schootbrugge et al. 2019).

For the Pliensbachian-Toarcian boundary interval, this scenario is applicable, as a warming pulse is postulated (associated with initial stage of the Karoo magmatism; Dera et al. 2009; Pieńkowski et al. 2016; Ruebsam et al. 2020). However, for the remaining part of the Tenuicostatum Zone many authors (e.g., Brandt 1986; McArthur et al. 2000; Guex et al. 2001; Suan et al. 2010; Dera et al. 2011; Korte and Hesselbo 2011; Krencker et al. 2014; Korte et al. 2015; Ruebsam et al. 2020; Ullmann et al. 2020) suggest overall persistent cool conditions, which should (according to the thermohaline-oceanic circulation mechanism) cause enhanced circulation—which is not observed in Mochras, where stagnation dominates. Only in a short interval between ~ 838 and ~ 853 mbs, is there a slight improvement of bottom conditions (confirmed also by foraminiferal assemblage; see Reolid et al. 2019), which could be associated with somewhat increased current intensity, possibly caused by a transient cooling, but this signal is weak.

Concerning the circulation problem, even more intriguing is the To-CIE interval, well known to be associated with the most severe carbon-cycle disruption, high pCO2 and extreme greenhouse conditions, which should cause even stronger diminution of southward flow and extreme sea-floor current stagnation at that time; instead, shortly after the beginning of the To-CIE (in our calculation ~ 200 kyr), the deep-sea circulation significantly accelerated. Only at the beginning of To-CIE did sluggish circulation and oxygen depletion persist, which is indicated by the continued Phycosiphon crisis and common occurrences of Trichichnus. For the remaining part of the To-CIE event, certainly from its climax (~ 820 mbs), gradually improving circulation can be interpreted. This is recorded by common current-generated structures (pin-stripe current lamination, occasionally ripple-cross lamination), some sharp boundaries and coarser sediments, and gradual return of more diversified trace fossils, such as Phycosiphon, Thalassinoides, Schaubcylindrichnus, Planolites, Lockeia, Asterosoma, Skolithos and Rhizocorallium, and concomitant demise of Trichichnus (Fig. 2a). The recovery is also visible in the foraminiferal assemblage (Reolid et al. 2019).

It is noteworthy that anoxia was suggested by some geochemical indices to be coincident with this ‘recovery’ phase of the To-CIE: Xu et al. (2018a) reported trace amounts of the biomarker isorenieratane (with slightly elevated gammacerane indices) at two horizons (811.66 mbs and 819.10 mbs) at and just above the peak of the To-CIE. Isorenieratane is a pigment of photosynthetic green sulphur bacteria, Chlorobiaceae, that is regarded as an indication for photic zone euxinia in the water column (Koopmans et al. 1996). We investigated these horizons in detail, finding that they contain bioturbated mudstones, either by meiofauna (Fig. 8e), or more distinctly by Phycosiphon incertum type 2 (Fig. 8h). This means that anoxic to euxinic conditions in the photic zone would not necessarily have affected the seafloor in Mochras, which was continuously oxygenated by deep-sea bottom currents. This observation is of more general significance, suggesting that geochemical indices (even if correctly pointing to certain conditions in a shallow-water column) may not always indicate euxinic bottom conditions, because deep-sea currents could deliver enough oxygen to sustain life there, even if the shallow-photic zone was anoxic. However, a scenario of bottom re-oxygenation (to the degree required by trace makers) and overprinting by bioturbation of sea floor that had been previously affected by anoxia is also possible—although this would still point to the short and transient nature of any oxygen crisis and, given the water depth and muddy productive nature of the water column, the sea-floor is unlikely to have been in the euphotic zone.

Observed lithological and ichnological fluctuations in the Toarcian are thus associated with alternating periods of vigorous currents and more stagnant conditions on various timescales, while nutrient availability seems to have been relatively stable and sufficient all the time (as shown by the TOC content; see Xu et al. 2018a) and played a subordinate role in setting ichnological trends. Hierarchical cyclic successions observed both in lithology and ichnology indicate that orbital forcing was still the main controlling mechanism. However, at the beginning of the Toarcian, the lithological and ichnological appearance of these cycles became different, which can be linked to non-orbital, intrinsic Earth mechanisms leading to prolonged ichnological crisis in the Tenuicostatum Zone. More generally, it is also observed that compared to the Pliensbachian, the energy of contour currents in the Toarcian (after the Tenuicostatum Zone) was overall higher, temporarily producing marked erosional surfaces and carrying coarser sediment, which collectively resulted also in less regular thicknesses of sedimentary packages corresponding to individual ~ 100 and ~ 405 kyr cycles (although the 1st order cycles retained approximately stable thicknesses, which point to a generally stable average sedimentation rate in a longer period of time). Although there is no clear correlation between the organic matter carbon isotopes and observed higher energy of currents, there is a return to generally lighter values of δ13C from the middle part of the Bifrons Zone.

Of note also is the repetition of the two fine-grained intervals characterized by particularly sluggish circulation, impoverished ichnodiversity (including the Phycosiphon crises), and abundance of Trichichnus: the lower one of latest Spinatum Zone to Tenuicostatum Zone age, and the upper one belonging to the Thouarsense and Dispansum zones. These intervals are separated by ~ 7.5 Myr in Mochras. A similar mega-cycle was observed in the Pliensbachian section (Pieńkowski et al. 2021), and comparable long cycles have also been hypothesised by Martinez and Dera (2015) and Charbonnier et al. (2023) and tentatively identified with a very long eccentricity term.

4.3 Climate changes and carbon cycle disturbances: impact on deep-sea circulation and orbital forcing

Two intrinsic Earth events should be considered carefully in terms of their influence on the Toarcian hemipelagic sedimentation in Mochras. The first was the Pliensbachian-Toarcian (Pl-To) boundary event, characterized by a CIE that is globally documented and linked to the initial phase of volcanism in the Karoo Province; the second event with profound impact on the marine and continental environment was the To-CIE (also known as the Jenkyns Event), dated to the latest Tenuicostatum Zone to Exaratum Subzone of the Serpentinum Zone, linked to large-scale eruptions in the Karoo-Ferrar basaltic province and associated with a rapid increase in atmospheric pCO2 levels and average atmospheric and marine temperatures (see e.g., Hesselbo et al. 2000, 2007; Littler et el. 2010; Hesselbo and Pieńkowski 2011; Bodin et al. 2016, 2023; Pieńkowski et al. 2016; Fantasia et al. 2018; Xu et al. 2018a; Ruebsam et al. 2020; Al-Suwaidi et al. 2022; Ruhl et al. 2022).

It is important to understand how these major environmental changes, indicated by disturbances in the carbon cycle, affected the deep-sea circulation. For a long time (since the Sinemurian) this circulation was controlled in a stable way by orbital forcing, expressed in cyclic fluctuations of southward flowing thermohaline contour currents in the Cardigan Bay Basin, particularly expressed in long-term trends governed by grand eccentricity (~ 2.4 Myr) cycles (Pieńkowski et al. 2021). Notably, Charbonnier et al. (2023) postulated shortened long-period cyclicities of ~ 1.6 and ~ 3.4 Myr in Early Jurassic orbital periods, possibly reflecting the chaotic orbital motion of the inner planets (likely corresponding to the Cenozoic 2.4 Myr and 4.7 Myr eccentricity terms). This is incompatible with our interpretation even if the 7–8 Myr very long eccentricity cycle and the Toarcian duration postulated both by Charbonier et al. (2023) and by us are in agreement. Possibly, in Mochras we would have to deal with the mutual modulation of the two cyclicities indicated by Charbonier et al. (2023), giving approximately the resultant cyclicity of 2.4–2.5 Myr observed in Mochras.

The 2.4 Myr cyclicity in the present work was directly observed in the core by visual scrutiny of the variability of sedimentological and ichnological parameters. In turn, long-period cyclicities were detected by Charbonier et al. (2023) based solely on the numerical spectral decomposition of magnetic susceptibility time series from the Sancerre-Couy borehole, Paris Basin, France. Such spectral techniques reveal component frequencies of basic orbital periodicities that directy affect the sedimentary record. Importantly, Pliensbachian grand orbital cycles in Mochras (Pieńkowski et al. 2021), put together on an interpreted 405 kyr cyclicity based on CaCO3 cyles (Ruhl et al. 2016), clay minerals (Deconinck et al. 2019), δ13C (Storm et al. 2020) and ichnology-sedimentology (Pieńkowski et al. 2021) also show prevalence (despite some discrepancies) of ~ 2.4–2.5 Myr cyclicity. This arrangement, based on direct ichnological and sedimentological observation, persists into the Toarcian at Mochras. Therefore, we favour effective influence of ~ 2.4–2.5 Myr cyclicity on sedimentary processes in Mochras, because adoption of the ~ 1.6 Myr cyclicity postulated by Charbonier et al. (2023) is less comparable to observed long-term sedimentary and paleoecological trends observed both for the Pliensbachian (Pieńkowski et al. 2021) and the Toarcian (this study). The possibility of blurring and distortion of orbital cyclicity by autocyclic depositional processes, or tectonic processes, particularly in shallower epicontinental environments such as at Sancerre, should not be totally ruled out in that context.

As shown by our data from the Toarcian of Mochras (Fig. 2a, b), the effects of orbital forcing were no doubt also influenced by the palaeoceanographic, palaeoenvironmental and tectonic factors. The picture obtained differs from surrounding epicontinental seas, showing also more complicated mechanisms of oceanographic processes, as a result of mutual influence of orbital forcing and non-orbital changes related to large-scale volcanism and climatic/environmental disturbances. The mechanism linking sluggish deep-sea circulation with climate warming, proposed by Dera and Donnadieu (2012), Ruvalcaba Baroni et al. (2018) and Van Schootbrugge et al. (2019), suggested for the Sinemurian–Pliensbachian section (Pieńkowski et al. 2021), appears to be still valid for most of the Toarcian in Mochras, but fails to explain changes that occurred in the earliest Toarcian (Tenuicostatum–Exaratum interval). Severity and continuity of the latest Pliensbachian to earliest Toarcian stagnation and benthic crisis probably occurred during transition from supposed icehouse to greenhouse conditions (Dera and Donnadieu 2012; Krencker et al. 2014; Bodin et al. 2023) and from sea-level lowstand to marked sea-level rise (Pieńkowski 2004; Haq 2017; Ruebsam et al. 2020; Bodin et al. 2023).

Stagnant conditions at the Pliensbachian–Toarcian boundary did not end at the beginning of the Tenuicostatum Zone, but continued through the whole zone (short-lived, slight improvement of oxygenation at the depth interval between 838.5 mbs and 847.0 mbs is too weak to be considered as significant break in the general stagnation). If the Tenuicostatum Zone was a cool period of time, then, according to the cooling/faster circulation paradigm, it would have resulted in a faster circulation throughout the entire zone. Instead, the Pliensbachian–Toarcian boundary event occurred at the beginning of long-lasting stagnation in Mochras. The sea-level rise postulated for this time interval (Pieńkowski 2004; Haq 2017; Ruebsam et al. 2020; Bodin et al. 2023) could have had some impact on deep-sea circulation, but it is uncertain in which direction. Sea-level rise would have led to lower energy of sedimentary processes and stagnation on continental shelves (due to the raised wave base), but it is unclear what would have happened in the case of much deeper water in the Cardigan Bay Basin. The influence of salinity drop in the early Toarcian epicontinental sea in UK (Remirez and Algeo 2020; Hesselbo et al. 2020b) could be considered as well, because density gradient is one of the main driving forces of deep-sea currents. By analogy with the recent Mediterranean and Atlantic waters (e.g., Bethoux et al. 1999), peri-Tethyan waters should have been more saline (due to evaporation) than waters in the Laurasian Seaway or Boreal Sea, where salinity could be diminished by freshwater influx from rivers and intermittent ice melt. However, the exact influence of the salinity gradient (which is elusive itself) on current intensity in the area studied remains uncertain.

The prolonged stagnation during the Tenuicostatum Zone could have resulted from coincidence of several overlapping causes, such as a warming phase of orbital forcing connected with a possible ~ 2.4 Myr grand eccentricity minimum, amplified by a concomitant warming effect, caused by initial Karoo volcanism emissions and consequent increase in pCO2. Due to the mutual feedback effect and inertia of deep-sea oceanographic processes, this stagnation would last for a long time (~ 800 kyr). Stagnation of deep-sea circulation in the Laurasian seaway may challenge interpretations of continued cold conditions during the Tenuicostatum Zone, although the data from Yorkshire are unambiguous on this point (Korte et al. 2015, their Fig. 2). On the other hand, the Cleveland Basin could be a local anomaly, as some other reports (e.g., Ruebsam et al. 2020, their Fig. 5) claim that during the Tenuicostatum (= Polymorphum) Zone the Earth’s climate system shifted between contrasting climatic conditions, particularly at the beginning and the end of this zone.

Extreme global warming occurred during the following Exaratum Subzone of the Serpentinum Zone, with benthic sea water temperature suggested to increase from ~  + 3 to 5 °C in mid-latitude Laurasian Seaway locations (see discussions in McArthur et al. 2000; Pálfy and Smith 2000; Gómez et al. 2008; Suan et al. 2010; Dera and Donnadieu 2012; Ullmann et al. 2020), or even as much as + 10 °C suggested for some sea-surface temperatures (Ruebsam et al. 2020). Such temperature increases, jointly with the demise of polar sea ice and stronger high-latitude continental runoff rates—which could have freshened the Arctic surface seawaters—would be expected to cause a general thermohaline circulation collapse (Dera and Donnadieu 2012). However, this is not evident in Mochras—instead, the Exaratum Subzone extreme global warming ended the previously dominating stagnation, counterarguing the previously supposed mechanism linking stagnation with warming. Likely, this time the global warming was so strong, that its consequences for deep-sea circulation were opposite—instead of sluggish circulation caused by northward currents only neutralizing dominating southward flow, the Tethyan northward flow became strong enough to prevail over the southward currents. As a consequence, the reversed deep-sea circulation became faster in the Cardigan Bay Basin. In particular, Dera and Donnadieu (2012) in their General Circulation Models (GCM) simulations found that for significantly higher pCO2 levels (> 1600 ppm), the depth of mixed layer appears deeper in the European basins. This suggests a good ventilation of the water column during the To-CIE, which is obviously not the case in semi-closed, epicontinental seas in western Europe. However, since this simulation was based on a homogeneous epicontinental bathymetry of 200 m, even more significant results could be expected with a deeper bathymetry of the Cardigan Bay Basin. Moreover, it is worth noting that for other periods such as the Late Permian, GCM simulations show that massive rises in atmospheric CO2 concentrations may drive more vigorous and more symmetrical deep-sea circulation under warmer climates (Winguth et al. 2002; Winguth and Maier-Reimer 2005).

According to Dera and Donnadieu (2012), the Exaratum Subzone thermal anomaly was probably related to a strengthening of warm equatorial Tethyan westward currents, drifting along the northern Gondwanan margins. Furthermore, the drift of these currents through westernmost areas and their subsequent clockwise rotation due to southward directed boreal flows is consistent with neodymium isotope data (Dera et al. 2009), as well as ammonite and nannofossil migration routes evidenced for the Early Toarcian (Reggiani et al. 2010; Dera et al. 2011). It is also likely that more vigorous NW Tethyan currents would account for major disruptions in faunal provincialism and northward expansion of marine Mediterranean faunas at the beginning of the Toarcian (Macchioni and Cecca 2002; Vörös 2002; Arias and Whatley 2005; Dera et al. 2011).

During the Tenuicostatum Zone to Exaratum Subzone period of time, the sedimentation rate in Mochras remained relatively stable and was still paced by precession (~ 20 kyr) and short- and long-eccentricity cycles (~ 100 and 405 kyr), which are all well expressed (Fig. 2a). Circulation decelerated just after the To-CIE event, in the earliest Falciferum Subzone (785–795 mbs), which is reflected by impoverished ichnodiversity and lithological features. From this time on, Toarcian circulation evidently became generally faster, particularly with regard to the middle parts of 1st order cycles, where more condensed sections and more frequent erosional/omission surfaces occur. The observations confirm that the warming/stagnation versus cooling/enhanced circulation mechanism returned after the To-CIE disturbances.

Importantly, existing data on sea-water temperature fluctuations in the Toarcian (McArthur et al. 2000; Gómez et al. 2008; Suan et al. 2010; Krencker et al. 2014; Ruebsam et al. 2020; Ullmann et al. 2020) support our conclusions regarding a large-scale correlation between sea-water temperature and inferred fluctuations of deep-sea current in the Cardigan Bay Basin. From the late Falciferum Subzone onwards, oxygenation of the sea floor generally improved; circulation intensity and average oxygenation achieved highest levels in the late Bifrons and Variabilis to earliest Thouarsense zones, as shown by sedimentological and ichnological features (Fig. 2b). This is likely associated with a long-term cooling trend, extending from the latest Falciferum Subzone to the Thouarsense Zone, with more intensified cooling in the Variabilis Zone. The latter was interrupted by a brief warming in the middle part of this zone (McArthur et al. 2000; Krencker et al. 2014). This brief warming seems to be also reflected in a slightly deteriorated ichnodiversity in Mochras (707–713 mbs), at the same time marking a boundary of ~ 2.4 Myr cycle.

The latest Thouarsense–Dispansum time interval in Mochras shows the next significant deterioration of ichnodiversity/oxygenation. This is likely connected to a warming trend that intensified in Dispansum Zone (Gómez et al. 2008), exactly when we are dealing with the conspicuous ichnodiversity/oxygenation crisis in Mochras. A weaker warming in the Pseudoradiosa Zone also corresponds to a slight deterioration of ichnodiversity around 610–615 mbs. The repetitive correlations between Toarcian sea-water temperature reconstructions elsewhere and contourite current intensity in the Cardigan Bay Basin point to a strong relationship between these variables, linking deep-sea circulation to major climate changes.

Toarcian sea-level changes are sometimes suspected as a possible factor in observed oceanographic processes. According to Ayranci et al. (2018), high-stand and transgressive-system tracts in Devonian contourites are represented by dominantly massive mudstone lithofacies, less intense bioturbation, and higher TOC values. Rising relative sea-level trends are confirmed for the Tenuicostatum Zone and also early Serpentinum, Bifrons and early Variabilis zones (Sellwood 1972; Graciansky et al. 1998; Pieńkowski 2004; Hesselbo 2008; Rocha et al. 2016; Barth et al. 2018; Haq 2017; Ruebsam and Al-Husseini 2021), but their relation to observed ichnological and sedimentological features in the Mochras profile is uncertain (Fig. 2a, b). Assuming a deep-sea setting of the Cardigan Bay Basin, major oceanographic/climate changes would have had a stronger impact than eustatic sea-level changes with amplitudes of only tens of metres (Haq 2017). Generally, a higher sea level might have enhanced the exchange of waters between adjacent basins and the circulation of currents, but the impact of eustatic (or tectonically-induced) sea level on deep-sea currents or the sediments entrained within them remains uncertain.

In addition to the beginning of the Toarcian (as discussed above), the relations between sedimentary (orbital) cycles and δ13C fluctuations are not as clear as they were in older Early Jurassic stages at Mochras (Storm et al. 2020). Some of the minor negative excursions at depths of 750, 719, 697, 675, 667 and 630 mbs seem to be related with more stagnant conditions (short-lived minor Phycosiphon crises, appearances of lamination and Trichichnus), but this relation requires more systematic study and a much higher resolution C-isotope curve. This is particularly important in the upper part of the profile, where the variations in δ13C values are subdued.

4.4 Duration of the Toarcian and ammonite zones based on ichnological astrochronology

The Toarcian section of the Mochras core represents the most expanded and highest resolution cyclostratigraphic dataset for this stage to date (even if much of the Aalensis Zone is eroded), and provides the most reliable basis for an astrochronological time scale, reflecting Milankovitch forcing, predominantly at precession/obliquity and the short- and long-eccentricity periodicities (Fig. 2a, b). The new results allow an estimation of the duration of the Toarcian stage at a minimum of 9.4 Myr (without most of the Aalensis Zone). Results were obtained from ichnological and sedimentological signals by counting successive 4th, 3rd and 2nd order cycles (interpreted respectively as precession/obliquity, short eccentricity, long eccentricity). Uncertainty values of zone durations are estimated based on depth-domain uncertainties of zonal boundaries (Additional file 3). For each zone, summed uncertainties of the base and top boundary are treated as a percentage of nominal zone thickness and proportionally converted to time domain. In case of the Dispansum and Pseudoradiosa zones the time-domain uncertainties are represented by spurious digits and are therefore negligible. Astronomical durations for the ammonite chronozones (based mainly on ~ 405 kyr, 2nd order cycles) are: Tenuicostatum Zone = 0.9 ± 0.1 Myr, Serpentinum Zone = 1.4 ± 0.4 Myr, Bifrons Zone = 2.1 ± 0.5 Myr, Variabilis Zone = 2.4 ± 0.1 Myr, Thouarsense Zone = 0.8 ± 0.1 Myr, Dispansum Zone = 0.3 ± 0.07 Myr, Pseudoradiosa Zone = 1.4 ± 0.02 Myr. The Aalensis Zone (truncated at its top) is preserved probably representing no more than 0.5 Myr of deposition. The obtained durations of successive ammonite zones (Table 3) are slightly longer compared to those shown in GTS2020 (Hesselbo et al. 2020a).

Table 3 Duration of Toarcian biochronozones

5 Conclusions

  1. 1.

    An integrated ichnological-sedimentological study of the Toarcian siliciclastic-calcareous contourite deposits in Mochras helps facilitate understanding of contourite deposition processes over a long period of geological time and its ichnodiversity. Together with the Pliensbachian part of the section (Pieńkowski et al. 2021), the current study comprises an expanded ichnological and sedimentary record of contourite deposits, adding new information on the sedimentary dynamics, variability of oceanographic history, and basin interconnectivity.

  2. 2.

    Ichnological signals point to a common and strongly repetitive mechanism driving the observed fluctuations in benthic conditions, which is indicated by spectral analysis and distinction of four hierarchical orders of cycles attributed to orbital forcing.

  3. 3.

    Long-term climate change controlled the circulation of contour-parallel bottom currents in the Cardigan Bay Basin and orbital forcing has set the timing for cyclic ichnological records, with the amplifying feedback of climate-related palaeoceanographic variations of thermohaline-driven contour currents circulating between the Boreal ocean and Peri-Tethys, through the Cardigan Bay Strait. There is continuity of ~ 2.4 Myr, rather than ~ 1.6 Myr cyclicity expressed in deep-sea circulation and the probable occurrence of yet longer, ~ 7–8 Myr (~ 7.5) periodicity which may or may not be under an orbital control. The mutual modulation of the two cyclicities (~ 3.4 and ~ 1.6 Myr) indicated by Charbonier et al. (2023), giving approximately the resultant cyclicity of 2.4–2.5 Myr observed in Mochras would be a plausible explanation, if the hypothesis of chaotic behaviour of the Solar System in Mesozoic is confirmed.

  4. 4.

    In more temperate climates, the formation of high-density waters was associated with low temperatures at higher latitudes and sinking of cooler and denser water masses, which then flowed south. However, in the early Toarcian this secular, extrinsic paradigm has been modified due to intrinsic Earth processes related to large-scale volcanism and higher temperatures, which led to intense evaporation and increase in salinity in the middle latitudes. This hypothetically began to play a dominant role in circulation, stopping or reversing the directions of marine currents, which resulted in a temporal disturbance of a long-term orbitally-controlled mechanism, but without eliminating the influence of orbital forcing.

  5. 5.

    The most severe oxygen crisis (dysoxic, but not anoxic), evidenced by trace fossils (Phycosiphon scarcity, mass appearances of Trichichnus) commenced shortly before the Pliensbachian-Toarcian boundary and ended at the early stage of the Toarcian negative carbon isotope excursion (To-CIE), which is in contrast to the general pattern of the T-OAE anoxic phenomenon in relatively shallow marine settings and its synchroneity with the To-CIE. The extreme climate warming, coeval to To-CIE, is therefore hypothesised to have caused significant changes in deep marine circulation, reversing its direction and improving oxygenation of the deep-sea floor.

  6. 6.

    Ichnological results, both macroscopic observations and spectral analysis, confirm the cycles that appear in the Mochras visual descriptions, and a new minimum estimate of duration of the Toarcian (9.4 Myr) and constituent ammonite zones is proposed.

  7. 7.

    Contourites of the Cardigan Bay Basin provide information on inter-ocean circulation, benthic conditions, and climate changes, which can be extracted using sedimentological and ichnological methods. The Cardigan Bay Strait played an important role in the Early Jurassic (at least Pliensbachian and Toarcian) marine circulation as a major link between the northern and southern part of the Laurasian Seaway, and in general between the Boreal and peri-Tethys oceanic domains.

  8. 8.

    Ichnological records in continuous hemipelagic successions seem to be a sensitive and reliable basis for the recognition of cyclic orbital forcing of climatic and palaeoceanographic conditions. Smoothed curves based on specific ichnotaxa occurrence time series bear resemblance to geochemical and geophysical proxy records, indicating concurrent response to the changing environmental conditions.

Availability of data and materials

All data generated during this study are included in the paper and its Additional files. Downhole log data from Mochras are available via the UK Onshore Geophysical Library (UKOGL) website



Toarcian negative carbon isotope excursion


Toarcian oceanic anoxic event


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The National Geological Repository of the British Geological Survey, Keyworth, Nottingham, UK, is acknowledged for sharing the Mochras drill core for investigations. There staff are warmly thanked for longstanding support of this project. Two anonymous reviewers provided corrections and helpul comments.


The research is financed by the National Science Centre, Poland, from the programme Opus 13, Grant Agreement No. 2017/25/B/ST10/02235 and the internal Polish Geological Institute Grant No. 62.9012.2016.00.0. This is a contribution to the ICDP and NERC JET Project: SPH and KNP were supported from the Natural Environment Research Council (NERC), Grant No. NE/N018508/1. For the purpose of open access, the author has applied a ‘Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising from this submission.

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GP: conceptualization, data curation, formal analysis, investigation, writing—original draft (lead); AU: conceptualization, data curation, formal analysis, investigation, funding acquisition (lead), project administration (lead from the National Science Centre, Poland side), writing—review and editing; KN: numerical analyses (lead), writing—review and editing (partial); KNP: refinement of ammonite biostratigraphy; SPH: conceptualization, review and editing, funding acquisition (partial), formal analysis, project administration (lead from the ICDP and NERC project JET sides).

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Correspondence to Alfred Uchman.

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Supplementary Information

Additional file 1

. Matrix of binary time series used for spectral analysis.

Additional file 2

. Description of time series and the runs test methodology and results.

Additional file 3

. Toarcian ammonite biozones for the Llanbedr (Mochras Farm) borehole, compiled from KN Page in Copestake and Johnson (2014) and McArthur et al. (2016); refer to Additional files 4 and 5 for details.

Additional file 4

. Biostratigraphically significant ammonite fossils from the Lower Toarcian of the Llanbedr (Mochras Farm) borehole, Wales, and proposed correlations to Yorkshire coast outcrops; all fossil identifications by KN Page.

Additional file 5

. Biostratigraphically significant ammonite fossils from the Upper Toarcian of the Llanbedr (Mochras Farm) borehole, Wales; all fossil identifications by KN Page.

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Pieńkowski, G., Uchman, A., Ninard, K. et al. Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes: Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK. Prog Earth Planet Sci 11, 18 (2024).

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